2. Determination of the Silicon
When wrought-iron, cast-iron and steels are dissolved in mineral acids, the
silicon present separates, after evaporation, as silica which may be collected
and weighed. In practice, the metal is treated sometimes with hydrochloric
and sometimes with nitric acid (in some cases sulphuric and nitric acids
together), according to the operations to be carried out subsequently on
the liquid. Thus, if only the silicon is to be determined or if the liquid
is to be used for estimations not affected by the presence of hydrochloric
acid, the latter is preferred owing to its more rapid action. In some cases,
however, the next estimation to be made with the liquid—e.g., that of
phosphorus—renders necessary the use of nitric acid. Both methods of
acting on the metal are in common use and will be described.1
1. Attack of the Metal with Hydrochloric Acid.—In a porcelain
dish covered with a clock-glass 2-4 grams of the sample 2 are heated with
hydrochloric acid of D 1-12 (about 10 c.c. per gram of metal) on the water-
bath until the iron is completely dissolved. The clock-glass is then removed
and washed into the basin and the liquid evaporated to dryness on the
water-bath, care being taken to mix with a platinum spatula the mass of
ferric chloride separating ; the residue is then heated for an hour in an
oven at 135°.
When cold, the dish is again covered with a clock-glass and the residue
moistened with 10 c.c. of hydrochloric acid (D 1-12), heated for some time
on the steam-bath, diluted with 100-150 c.c. of hot water, mixed and, after
cooling, filtered, the residue being washed with cold water acidified with
hydrochloric acid until the wash water is free from iron. At this point a
few drops of hydrochloric acid (D 1-12) are allowed to flow down the edge
of the filter, which is again washed with cold distilled water.
The moist filter is placed point upwards in a tared platinum crucible
and carefully burnt, the crucible being ignited for 4-5 minutes in the blow-
pipe flame and, when cold, weighed.
The silica thus separated may contain various impurities, such as graphi-
tic carbon, traces of ferric oxide, tungsten oxide, titanic acid, etc.
To determine the true silica content, the weighed residue is treated in
the crucible with 1-2 c.c. of water, 1-2 drops of cone, sulphuric acid and
5-6 c.c. of hydrofluoric acid (puriss.). Evaporation is then carried as far
as possible on the steam-bath and the slight excess of sulphuric acid expelled
by heating the inclined crucible over a small flame. When evolution of
sulphuric acid vapour ceases, the crucible is heated to redness and weighed
when cold. The loss in weight gives the silica and this, multiplied by
0-4693, the silicon.
It sometimes happens, especially with cast-iron rich in graphite, that
the silica separated is grey. In this case, the treatment with hydrofluoric
acid is omitted and the contents of the crucible heated to gentle fusion
with a little sodium carbonate and potassium nitrate. When cold, the mass
1 For the determination of silicon in products insoluble in acid, see Ferro-silicon.
2 With samples containing little silicon, such as ordinary carbon steels, 5-10 grams